It applies in particular to a free space laser optical communication terminal.
Free space laser optical communication is used for transmitting data between two points which may move relative to one another. For example, it may be used for a communication between two artificial satellites, between a satellite and an aircraft, a satellite and a ground terminal, etc.
In this transmission method, two terminals that are in mutual communication each produce a laser beam towards the other terminal. Each terminal is simultaneously a transmitter and a receiver, so that the data may be transmitted in both directions between the two terminals. The data are encoded in an appropriate manner for being transmitted in the form of laser beam pulses. Even if one of the terminals has no data to transmit momentarily, it nevertheless generates a beacon signal which is used for the pointing of the other terminal.
An optical communication terminal therefore comprises a laser-signal transmission system which produces and transmits such signals in a transmission direction. It also comprises a reception system which makes it possible to detect laser signals which originate from a determined direction, called the reception direction. Variations in this reception direction are caused by the relative movement of the two terminals during the communication connection, and by changes in attitude of the carrier on which the terminal is used. These variations are detected continuously by changes in the focusing point of the received laser signals on a matrix of photodetectors which is included in the reception system. The transmission direction of the terminal is then adjusted according to the variations in the reception direction, so that the laser beam of the transmitted signals passes through the reception pupil of the other terminal. Such pointing operation of a terminal occurs during the communication phase which corresponds to the transmission of the data. This phase is commonly called the tracking step. During this step, the pointing of each terminal is maintained in the direction of the other terminal despite the relative movement of the two terminals and their respective attitude movements.
More precisely, the transmission direction is computed based on the reception direction by adding to the latter an angular deviation and an angle of forward pointing, often referred to as the point-ahead angle. In a known manner, the angular deviation compensates for a lack of parallelism between the transmission and reception directions. The point-ahead angle corresponds to the relative movement of the two terminals during the round-trip transmission of the laser signals. It is computed by combining, in a reference inertial frame, the propagation speed of the laser signals with the respective speeds of the two terminals.
The co-alignment angle, i.e. the angle between emission and reception directions is measured after the terminal has been placed in its operating situation, for example once the satellite on board which it is installed is in its final orbit. This measurement is usually repeated before each communication session, in order to get rid of the deformations due to the temperature changes of the terminal. A retractable reflector is then placed temporarily at the exit of the transmission system in order to send to the reception system a part of the beam that is generated by this transmission system. The detection of this part of the transmission laser beam on the photodetector matrix makes it possible to measure the co-alignment angle.
When one and same entrance optical system is used within a terminal both for collecting the laser signals that are received and for transmitting the laser signals that are produced by the transmission system, the reflector is a retroreflector which can be placed between this entrance optical system and an optical separating system of the respective paths of the received signals and the transmitted signals. Such a separating system separates the optical path of the signals that are received, in the direction of the photodetector matrix, from the optical path of the signals that are transmitted and that originate from the transmission source inside the terminal. In a known manner, such a retroreflector may advantageously be formed of three planar mirrors that are arranged like a corner cube. Indeed, the direction of reflection of such a retroreflector does not vary according to an involuntary inclination of the retroreflector.
The co-alignment angle is measured when the reflector is placed in the path common to the transmitted and received signals, then the reflector is withdrawn once the measurement is complete. Such a measurement of the angular deviation cannot be performed during a tracking step, because the reflector at least partially obscures the optical entrance field of the terminal. The value of the angular deviation that has been measured is then used during a fixed tracking duration.
But the exact value of the co-alignment angle may vary continuously, for example under the effect of temperature changes. A pointing error then results from the use of one and same co-alignment angle value for a prolonged duration. Periodically repeated measurements of this angle would reduce the time that is actually available for data transmission.
To use continually an exact value of the angular deviation, document U.S. Pat. No. 5,517,016 proposes permanently reflecting a part of the signals that are transmitted by the terminal to the photodetector matrix of the reception system. In this manner, the actual value of the angular deviation can be measured permanently during the pursuit step. But the part of the transmitted signals that is reflected is superposed, on the photodetector matrix, on the signals that originate from the partner terminal in the communication connection. Errors result from this in the decoding of the received data.